1. Field of the Invention
The present invention relates to a method for manufacturing a bonded wafer.
2. Description of Related Art
There have been suggested SOI which is also known as Silicon on Quartz (SOQ), Silicon on Glass (SOG) or Silicon on Sapphire (SOS), wherein a handle substrate is made of a transparent insulating substrate, and a bonded wafer which is obtained by combining a handle substrate formed of silicon or the like with a transparent wide-gap semiconductor such as GaN, ZnO, diamond or AlN (in this case, the semiconductor substrate is transparent). These are expected to be applicable to various applications. The SOQ, SOG, SOS and the like are expected to be useful in applications such as projectors and high frequency devices because of the insulating properties, transparency and the like of the handle substrate. Furthermore, a bonded wafer in which a wide-gap semiconductor film is placed on a handle substrate, contains a highly expensive wide-gap semiconductor material only in an amount of forming a thickness of several hundred nanometers to several micrometers so that there is a possibility for promoting significant cost reduction. Therefore, the bonded wafer is expected to be applicable to high performance lasers, power devices and the like.
The conventional SOI production technology associated with bonding is divided mainly into two kinds of methods.
One of the methods is a SOITEC method, by which a silicon substrate (donor substrate) that has been implanted with hydrogen ions and a substrate (handle substrate) that serves as a supporting substrate are bonded at a room temperature, subjected to a heat treatment at high temperature (around 500° C.) to generate a large number of micro air bubbles called microcavities at the ion implantation interface, and subjected to detachment for transferring a silicon film onto the handle substrate.
The other is a method called a SiGen method, by which a silicon substrate that has been implanted with hydrogen ions and a handle substrate are respectively plasma-treated for activating their surfaces, bonded, and subjected to mechanical detachment at the hydrogen ion implantation interface.
However, since combination of these materials involves bonding of different kinds of substrates, the coefficients of thermal expansion of the semiconductor substrate and the donor substrate do not coincide. In regard to the SOITEC method, since the heat treatment at high temperature (about 500° C.) for thermal detachment at the hydrogen ion implantation interface is carried out after bonding, there is a drawback in the case of bonding different types of substrates as described above, that the substrates develop cracks because of a large difference in the coefficient of thermal expansion. Furthermore, in regard to the SiGen method, bonding the surface-activated surfaces results in a higher bonding strength as compared with the SOITEC method, and a high bonding strength is obtained by a subsequent heat treatment at a relatively low temperature of about 250° C. to 350° C. However, through the course of experiments to arrive at the present invention, it has been found that when substrates bonded at room temperature are heated to the temperature range of about 250° C. to 350° C., the bonded substrates cause breakage or defects such as an untransferred portion because of the difference between the coefficients of thermal expansion of the two substrates. Furthermore, because a heat treatment suitable for making the ion implantation interface brittle is required, it is not desirable to avoid a heat treatment at 250° C. to 350° C.
As mentioned above, there are problems that substrates may be broken because of a difference in the coefficient of thermal expansion of the bonded substrates, or an untransferred portion of the silicon film occurs as the silicon film is transferred. This is because, although the bonding strength of the bonding interface increases as the temperature increases, simultaneous bonding of substrates of different types causes a warp so that detachment or the like occurs and the bonding does not proceed uniformly in the plane. When these substrates are bonded and then directly subjected to a high temperature treatment, there are problems such as the occurrence of substrate breakage, or detachment of the bonded substrates.
Therefore, owing to a difference in the coefficient of thermal expansion between the semiconductor substrate and the handle substrate, there is a drawback that it is difficult to employ a high temperature process (about 500° C.) for thermal detachment at the hydrogen ion implantation interface, which is carried out after bonding, and application of conventional methods represented by the SOITEC method is difficult.
Examples of the prior art of the above mentioned type can be found in Japanese Patent No. 3048201 and U.S. Pat. Nos. 6,263,941; 6,513,564 and 6,582,999.